The invention relates to the downhole generation and recording of seismic waves for use in investigation and monitoring of earth formation reservoir characteristics surrounding a wellbore. In particular, the invention relates to a method and system for monitoring seismic energy radiated from wellbores into surrounding earth formations. The seismic body waves radiated into the surrounding earth formation, which may be converted from borehole tube waves, are used for cross well type projects and reverse VSP type projects to investigate and monitor hydrocarbon or other mineral deposits over the productive lifetime of a producing reservoir.
This is a system for monitoring the distribution of the contents of a subsurface mineral deposit over its economic life span for long-term resource management. The system employs acoustic waves generated when borehole tube waves impinge upon minor borehole obstructions. Time varying changes of selected attributes of those acoustic waves that have transited the deposit between boreholes, may be indicative of the temporal changes in the mineral content.
Geophysical surveys are used to discover earth structure, mineral deposits, and the subsurface extent of mineral deposits such as oil, natural gas, water, sulphur, etc. Geophysical methods may also be used to monitor changes in the deposit, such as depletion resulting from production of the mineral over the economic lifetime of the deposit. The usefulness of a geophysical study depends on the ability to quantitatively measure and evaluate some geophysical analogue of petrophysical parameters related to the presence of the mineral under consideration.
Seismic methods may be applied to production-management monitoring as well as to exploration. As is well known to geophysicists, an acoustic source at or near the surface of the earth is caused periodically to inject an acoustic wavefield into the earth at each of a plurality of source survey stations. The wavefield radiates in all directions to insonify the subsurface earth formations. The radiated wavefield energy is reflected back to be received by seismic sensors (receivers) located at designated stations also usually located at or near the surface of the earth, but which may also be in the subsurface. The seismic sensors convert the mechanical earth motions, due to the reflected wavefield, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing.
The travel-time lapse between the emission of a wavefield by a source and the reception of the resulting sequence of reflected wavefields by a receiver is a measure of the depths of the respective earth formations from which the wavefield was reflected. The relative amplitudes of the reflected wavefields may be a function (an analogue) of the density and porosity of the respective earth formations from which the wavefields were reflected as well as the formations through which the wavefields propagated. The phase angle and frequency content of returned signals in the reflected wavefields may be influenced by formation fluids, the sought-for minerals or other formation characteristics.
The processed seismic data associated with a single receiver are customarily presented as a one-dimensional time scale recording displaying rock layer reflection amplitudes as a function of two-way wavefield travel time. A plurality of seismic traces from a plurality of receivers sequentially distributed along a line of survey at intervals, such as 25 meters, may be formatted side by side to form a two dimensional (2-D) analog model of a cross section of the earth. Seismic sections from a plurality of intersecting lines of survey distributed over an area of interest provide three-dimensional (3-D) imaging. A series of 3-D surveys of the same region made at successive time intervals, such as every six months, would constitute a 4-D, time-lapse study of the subsurface that would be useful to monitor, for example, the fluid-depletion rate of an oil field.
The term xe2x80x9csignaturexe2x80x9d as used herein, means the variations in amplitude, frequency an phase of an acoustic waveform (for example, a Ricker wavelet) expressed in the time domain as displayed on a time scale recording. As used herein the term xe2x80x9ccodaxe2x80x9d means the acoustic body wave seismic energy imparted to the adjacent earth formation at a particular location. The coda associated with a particular seismic energy source point or minor well bore obstruction in this invention will be the seismic signature for that seismic energy source point. The term xe2x80x9cminor borehole obstructionxe2x80x9d or xe2x80x9cborehole discontinuityxe2x80x9d or xe2x80x9cdiscontinuityxe2x80x9d means an irregularity of any shape or character in the borehole such that tube wave energy transiting the wellbore will impart some energy to the irregularity in the borehole and thus radiate body wave energy into the surrounding earth formation while also transmitting and reflecting some the tube wave energy as well. The term xe2x80x9cimpulse responsexe2x80x9d means the response of the instrumentation (seismic sensors and signal processing equipment) to a spike-like Dirac function or impulse. The signal energy of an acoustic wavefield received by seismic sensors depends upon the texture of the rock layers through which the wavefield propagated, from which it was reflected or with which it is otherwise associated, whether along vertical or along lateral trajectories. The term xe2x80x9ctexturexe2x80x9d includes petrophysical parameters such as rock type, composition, porosity, permeability, density, fluid content, fluid type and inter-granular cementation by way of example but not by way of limitation.
From the above considerations, it is reasonable to expect that time-lapse seismic monitoring, that is, the act of monitoring the time-varying characteristics of seismic data associated with a mineral deposit such as an oil field over a long period of time, would allow monitoring the depletion of the fluid or mineral content, or the mapping of time-varying attributes such the advance of a thermal front in a steam-flooding operation.
Successful time-lapse monitoring requires that differences among the processed data sets must be attributable to physical changes in the petrophysical characteristics of the deposit. This criterion is severe because changes in the data-acquisition equipment and changes in the processing algorithms, inevitable over many years may introduce differences among the separate, individual data sets from surveys that are due to instrumentation, not the result of dynamic reservoir changes.
In particular, using conventional surface exploration techniques, long-term environmental changes in field conditions such as weather and culture may affect the outcome. If time-lapse tomography or seismic monitoring is to be useful for quantitative oil-field reservoir monitoring, instrumentation and environmental influences that are not due to changes in reservoir characteristics must be transparent to the before and after seismic data sets. Successful time-lapse tomography requires careful preliminary planning.
One way to avoid many time-dependent environmental changes and updated state of-the-art instrumental changes is to permanently install seismic sources and seismic detectors in one or more boreholes in and around the area of economic interest. Identical processing methods are applied to the data throughout the monitoring period using cross-well (cross-borehole) tomography rather than conventional surface type operations. One such method is disclosed in patent application Ser. No. 08/949,748, filed Oct. 14, 1997 now U.S. Pat No. 5,886,255 and assigned to the assignee of this invention and which is incorporated herein by reference as a teaching of cross-well tomography.
U.S. Pat. No. 5,406,530, issued Apr. 11, 1995 to Tokuo Yamamoto, teaches a non-destructive method of measuring physical characteristics of sediments to obtain a cross sectional distribution of porosity and permeability values and variations and of shear modulus and shear strength. A pair of boreholes has borehole entries spaced apart from each other at a predetermined distance and a plurality of hydrophones is spaced at predetermined known locations. A pseudo random binary sequence code generator as a source of seismic energy is place in another borehole and activated to transmit pseudo random wave energy from the source to the hydrophones. Seismic wave characteristics are measured in a multiplicity of paths emanating from the source to the hydrophones using cross-well tomography.
The Yamamoto teaching is primarily directed to use in shallow boreholes for engineering studies. Such holes are less than 100 meters deep, as opposed to oil-field boreholes, which may be two to five kilometers deep. The requirement for an active source to be placed at various levels in the borehole is problematic because the source can damage the hole and interfere with production. Since the seismic equipment must be moved up and down the boreholes, it is impossible to maintain identical recording conditions over an extended time period.
G. W. Winbow in U.S. Pat No. 4,993,001 issued Feb. 12, 1991, describes a method and apparatus for converting tube waves into downhole body waves for seismic exploration. The equipment comprises a rotary-valve tube wave source for producing swept-frequency tube waves that are injected into tubing or well bore fluid. The tube waves are converted into body waves by an elongate tube wave converter located at a selected position downhole. The tube wave converter comprises an elongate body that preferably substantially fills the well bore or tubing and has a preferred shape in order to convert efficiently the tube waves to body waves at the selected position downhole. This patent is directed primarily to reverse VSP. Winbow acknowledges that it is well known in the art that xe2x80x9cnonuniformities in the boreholexe2x80x9d cause seismic-wave mode conversions that cause secondary acoustic radiation and associated multiples.
Winbow employs a single tube-wave converter to serve as a single source of direct and reflected seismic waves but he must repeatedly reposition the device at spaced-apart intervals down the length of the borehole to get extended vertical coverage as in cross-well tomography. That system thus is difficult to implement for the fixed permanent instrumental installation required for 4-D seismic monitoring operation.
There is a need for a system of seismic sources fixed permanently in boreholes that may be used for monitoring time-varying reservoir attributes such as the distribution of the contents of a mineral deposit. Additionally, there is a need for a system that not only would be used with intentionally generated seismic energy, but could also take advantage of naturally occurring or ambient energy in boreholes, energy that creates tube waves that may be converted to seismic body waves radiated into earth formations around boreholes.
This is an acoustic system and method for monitoring the radiation of a seismic wavefield into an earth formation surrounding at least a first source borehole containing a column of wellbore fluid. The energy source for the radiated seismic waves may be naturally occurring or intentionally provided energy that produces borehole tube waves that are converted to body waves are discontinuities in borehole tubulars. A plurality of arbitrarily spaced borehole discontinuities or minor obstructions or irregularities is present axially in a borehole fluid column. Each borehole discontinuity or minor borehole obstruction forms a point source for diffractively radiating an acoustic wavefield into the surrounding formation whenever any one of said discontinuities or minor obstructions intercepts the tube wave as the tube wave transits the column of well bore fluid. The resulting plurality of acoustic wavefields, each exhibiting a unique waveform coda, is thereby radiated into the formation. An acoustic sensor in association with one or more said discontinuities or minor borehole obstructions detects the unique waveform coda characteristic of each of the respective wavefields to provide a plurality of uniquely encoded pilot signals. A plurality of sensors for detecting seismic energy separated from the source borehole by an intervening earth formation is provided. The plurality of seismic receivers for receiving the plurality of acoustic wavefields radiated from the source borehole after the wavefields have propagated through the intervening earth formation may be in another borehole or on the surface of the earth. A cross correlator is provided for cross-correlating the respective uniquely encoded pilot signals with a corresponding received acoustic wavefield to furnish a cross correlogram that is indicative of the current value of the pre-selected attribute. A time-lapse profile is created that is representative of the temporal changes in the petrophysical characteristics and mineral content or distribution of the intervening earth formation by repeating the process at subsequent time intervals.